Because when a person is happy, that person is more likely to live longer.

Because you will have stronger immune system

Because happiness is contagious, you are promoting world peace.

Because happy people are more likely to become socially interactive.

More likely to be emotionally healthy

More energetic and more productive

More likely to exhibit greater self-control and coping abilities

More likely to look good.

Because your family friends, or lover wants you to be happy.

Because when people die, people wanted to be remembered as a happy person, rather than a grumpy one. (When a happy person dies a lot of people will be sad, but when a grumpy person dies, a lot of people will be happy